a) Field of the Invention
The invention is directed to a device for combining optical radiation by taking advantage of the polarization properties of light, in particular for medical equipment.
b) Description of the Related Art
Large areas of the use of lasers as radiation sources in the visible or invisible part of the spectrum in medicine and technology require the optical marking of the zone that is being worked on by means of a targeting or marking laser, the wavelength of the targeting laser normally being in the visible part of the spectrum. An active beam of an active beam laser is, for example, utilized for the irradiation of diseased areas, for example inside the eye. The active laser beam and the targeting laser beam are usually combined by means of dichroic optical elements, and/or parts of the radiation are decoupled from the active beam which amongst other things are used for measuring purposes.
Beam-splitter cubes or partially reflective plane-parallel plates arranged, for example, at an angle of 45xc2x0 in the corresponding beam path are used as such optical elements. These elements usually have a dielectric coating for the purpose of reducing reflection losses of their optically active surfaces on the wavelength of the active beam. In contrast, the coupling-in of the marking laser is done via dielectric coatings with a high degree of reflection on the surfaces. Reflected parts of the active beam caused by the combining optics can be used for power control and power monitoring of the active beam.
One such element is described in DE 19816302 C1 in connection with a device for the radiotherapy of tissue parts. For this device, a targeting beam and an active or therapy beam are combined by an optical element represented by a plane-parallel plate. This element has two optically active surfaces on which the combination of the beams and the decoupling of at least one partial beam from the active beam take place. The decoupled partial beams are directed onto photodectors, the signals from which are used for control or monitoring purposes after they have been further processed appropriately.
The disadvantages of this prior art can described approximately as follows: It requires modifications of the optical surfaces of the corresponding optical elements for the reduction of reflection losses and coupling losses. The realization is done by means of the application of dielectric layers which at the same time as reducing the degree of reflection of the surfaces on the wavelength of the active beam also increase the degree of reflection of the marking wavelength, but dielectric layers have the disadvantage that their optical properties change depending on environmental conditions. A change of environmental temperature or of air humidity can cause a marked change of the degree of reflection of the layer, the proportion of scattering losses and the wavelength characteristics. For the use of a coupling element according to prior art, this can lead to losses of power, wrong measurements, monitoring gaps and, in the case of application in medical equipment, ultimately to people being endangered. To minimize sensitivity towards environmental influences, optical wedges or prismatic elements are cemented on to ensure the orthogonality of the incident beam and the optical surfaces, but these measures have as a consequence the disadvantages that optical aberrations, like astigmatism, are caused by the coupling element, and complex dielectric coatings are still necessary on the coupling element, which increases production complexity.
Therefore, it is the primary object of the invention to provide a device for combining optical radiation from which the disadvantages of prior art have been removed to the largest extent and for which a combination of active beam and marking or targeting beam as well as the decoupling of beam parts are attained with minimal coupling losses by means of a constructionally simple optical coupling element by utilization of the polarizing properties of light.
In accordance with the invention, a device for combining optical radiation by utilizing the polarization properties of light is disclosed which comprises a plane-parallel optically transparent plate with a refraction index n which has an optically active first surface on which a first optical beam (active beam) impinges and a second optically active surface parallel to the first one on which a second optical beam (targeting beam) impinges on the exit location of the first beam from said second surface. The plane-parallel plate is arranged relative to the beams or the beam paths to be coupled in such a way that the first beam impinges on the first surface of the plate and the second beam impinges on the second surface of the plate and that both do this at an angle xcex1 which is equal to or approximately equal to the Brewster angle corresponding to the refractive index n of the plate. The first and second beams are linearly polarized. The plane of polarization of the first beam is parallel to the plane of incidence of the device and the plane of polarization of the second beam is orthogonal to the plane of incidence of the device.
So that the first and the second beam take the same course and have identical paths, it is useful for the second beam, which advantageously is the targeting or marking beam, to be brought together collinearly with the first beam at the exit location of the first beam from the second surface of the plane-parallel plate.
It is furthermore advantageous if a first partial beam is generated through reflection at the location of entry of the first beam into the plane-parallel plate, if a second partial beam is generated through reflection at the location of the exit of the first beam on the plane-parallel plate and if these partial beams are allocated to photodetectors, these being connected with a processing unit for generating control signals. By means of these split off partial beams, corresponding control signals can be generated which can be used for controlling or regulating the active and/or the targeting beam.
It is furthermore advantageous if polarization, absorption, holographic or dielectric filters lie in front of the photodetectors in the beam path of the partial beams.
The main advantage of the invention results from the fact that reflection reducing dielectric layers on the optically active surfaces of the plane-parallel plate are not used. On the one hand, cost and time required for the coating operation can be saved, on the other hand, all problems that occur in connection with dielectric layers no longer apply. This means that changing environmental conditions can not lead to a change of the layer properties. Since the degree of reflection of the optical surfaces only depends on the refractive index of the glass material, on the angle of incidence and, to a very low degree, on the divergence of the laser, the system proves itself to be invariant towards changing environmental conditions. In the case of the suggested arrangement, a highest possible independence of the influence of humidity and temperature is achieved.